net: wwan: t7xx: Add fastboot WWAN port

On early detection of wwan device in fastboot mode, driver sets
up CLDMA0 HW tx/rx queues for raw data transfer and then create
fastboot port to userspace.

Application can use this port to flash firmware and collect
core dump by fastboot protocol commands.
E.g., flash firmware through fastboot port:
- "download:%08x": write data to memory with the download size.
- "flash:%s": write the previously downloaded image to the named partition.
- "reboot": reboot the device.

Link: https://android.googlesource.com/platform/system/core/+/refs/heads/main/fastboot/README.md

Signed-off-by: Jinjian Song <jinjian.song@fibocom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net: wwan: t7xx: Add sysfs attribute for device state machine

Add support for userspace to get/set the device mode, device's state
machine changes between (unknown/ready/reset/fastboot).

Get the device state mode:
- 'cat /sys/bus/pci/devices/${bdf}/t7xx_mode'

Set the device state mode:
- reset(cold reset): 'echo reset > /sys/bus/pci/devices/${bdf}/t7xx_mode'
- fastboot: 'echo fastboot_switching > /sys/bus/pci/devices/${bdf}/t7xx_mode'
Reload driver to get the new device state after setting operation.

Signed-off-by: Jinjian Song <jinjian.song@fibocom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net: wwan: t7xx: Add maintainers and documentation

Adds maintainers and documentation for MediaTek t7xx 5G WWAN modem
device driver.

Signed-off-by: Ricardo Martinez <ricardo.martinez@linux.intel.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>